The use of aluminum alloys in the manufacture of automobile bodies and components has increased in part due to the need to reduce the weight of the vehicles for improved fuel economy. One application for aluminum alloys in the manufacture of automobiles is in the forming of body panels from aluminum alloy sheet stock. For example, hoods, doors and deck lids are formed by stamping an inner panel and an outer panel from suitable aluminum sheet stock. The outer panel forms the decorative and functional outline of the vehicle panel. The inner panel serves a reinforcing function. In the manufacture, then, of such a two-layer construction, the outer sheet is provided with suitable flanges at its edges. The inner sheet is laid against the outer sheet within the flanges, and the flanges are bent against the inner sheet in a hemming operation.
A series of aluminum sheet alloys have been developed which are strong and hard due to the presence of precipitated, finely divided hardening particles. One such series is the AA2XXX series in which small amounts of copper and magnesium, for example, are added to the aluminum alloy to contribute to hardening particle formation. Another series is the AA6XXX series where silicon, magnesium and copper are added for hardening. A third series is the AA7XXX series where zinc, magnesium and copper, for example, are added as hardening constituents. These alloys are well known and commercially available. They are formed into sheet stock from cast billets by a suitable sequence of hot rolling and cold rolling operations. Usually at the finish of the sheet forming/rolling operation, the sheet material is heated to dissolve in solid solution the small amounts of prospective hardening particles or transition phases such as Mg.sub.2 Si or GP zones (e.g., in the 6XXX series) and the like. The sheet is then quenched to retain such secondary phases in an unstable solution. The quenched material may be allowed to age at room temperature, whereupon the dissolved hardening constituents slowly reprecipitate in a very finely divided state to strengthen and harden the sheet. Such room temperature-aged alloys are usually identified as having a T4 temper designation. In some cases, the alloy is reheated after the quenching operation to induce reprecipitation of the hardening phases. The alloy is then designated as being in a T6 temper condition. The T6 alloys are usually stronger and harder than the T4 alloys. The terms "age hardening" and "precipitation hardening" are used interchangeably herein to include aluminum alloys aged at room temperature and alloys heated above room temperature to accelerate or increase the strengthening and hardening effect.
Thus, when an automobile body panel is formed from an aluminum alloy such as AA6111-T4, it is in an age-hardened condition. The properties of the alloy are a compromise which enable it to undergo suitable stamping and drawing operations and the like for shaping into a body panel and yet provide suitable strength and hardness in the finished panel.
A difficulty is that such age-hardened alloys, for example, the AA 2XXX, 6XXX and 7XXX series, are not sufficiently ductile to undergo all desired forming operations such as the above-described hemming operation that are desirable in the formation of a class A finish automotive exterior body panel. It is found that in the severe forming of such sheet alloys as, for example, where a sheet edge is bent flat against itself in a straight line hemming operation, cracks form in the outer bent surface of the hemmed sheet which detract from its appearance, its strength and its corrosion resistance.
Accordingly, it is highly desirable to have a method by which such hemming operations and other like severe bending or folding operations can be conducted on a precipitation-hardened aluminum alloy sheet without forming cracks or other defects in the bent or folded body.